In the middle of the Pacific Ocean, Hawaii Island is surrounded by thousands of miles of thermally stable seas. The 13,796-foot Maunakea mountain summit has no nearby ranges to roil the upper atmosphere, and for most of the year, this atmosphere is clear, calm, and dry, enabling the W. M. Keck Observatory, with its twin 10-meter-mirror telescopes, to observe our galaxy and beyond at levels special to it since opening in the early 1990s. Now, after the completion of a significant nine-year motion control upgrade project, the Keck Observatory telescopes, each standing 30 meters (almost 100 feet) tall, are offering data and observations with new and impressive nanometer precision. And all changes were made without experiencing any downtime on either telescope.
“We are now able to blindly point the telescopes to any star in the sky within Keck’s observable area with an accuracy of 1.0 arcsec,” explained Tomas Krasuski, Lead Electronics Engineer at Keck Observatory, who played an important role in the design and implementation of the telescope upgrade project that began in 2009. “That is an accuracy level of one thirty-six-hundredth of one degree.
“This, along with some of our other new, impressive performance numbers, is allowing researchers to gather data more accurately and quickly than ever before. New subsystems designed, developed, and now completed at Keck have enabled this level of accurate motion control, including the redesigns of our very critical azimuth and elevation systems that incorporate ultra-precise HEI-DENHAIN encoders, allowing true nanometer-level measurements that we did not have in the early ‘90s.”
Since the Beginning
The twin Keck Observatory telescopes are the most scientifically productive optical and infrared telescopes in the world. Each telescope weighs more than 300 tons and hosts a primary mirror with a 10-meter diameter (32.8 feet). These telescopes are currently the two largest light-collecting mirrors on the planet, operating at impressive nanometer-level precision.
“We are an extremely important resource for researchers interested in many areas of astronomy and astrophysics including the discovery of exo-planets; the study of how planets, stars, and galaxies form; the nature of black holes; and the chemical composition and evolution of the universe,” explained Krasuski.
When Keck Observatory began science operations in the early 1990s, it was the first generation of very large ground-based optical/infrared telescopes with segmented primary mirrors. The telescopes worked very well utilizing technology available at that time, though after 20 years, some replacement components became hard to find, putting the telescopes at risk in the event of a major failure. Also, the established measurement system included rotary encoders that were subject to periodic errors. So, following an obsolescence study, the need for renovations became clear.
When Keck Observatory’s Telescope Control System Upgrade (TCSU) project launched in 2009, it set out to not only update the systems but also improve the telescope pointing, tracking, and offsetting performance. Over the next nine years, both the Keck I (K1) and Keck II (K2) telescopes had upgrades done on all major elements including telescope controls, rotator and secondary mirror controls, and safety systems. And due to a switching solution developed and implemented for use between old and new control systems, the TCSU team was able to complete the specialized upgrade work with new components during the day, though at night, the established systems were switched back and still operational.
“TCSU was a complex and challenging project that involved multiple subsystems. Our team decided to upgrade all at the same time instead of consecutively to reduce the need for regression and repeated cross-compatibility testing,” explained Krasuski. The whole TCSU team was involved: Ean James, Ben McCarney, Kevin Tsubota, and Shui Kwok, as well as the summit maintenance crew. “This project would not be possible without the talent and expertise of this team,” said Krasuski. “We faced some tough challenges, but our collective hard work and determination helped us overcome those obstacles.”
At the beginning of the TCSU project, Keck Observatory engineers explained that a significant part of the project was the installation of new telescope azimuth and elevation position encoders based on HEIDENHAIN’s 40-micron grating tape scales. Interpolated to a 10-nanometer resolution with a HEIDENHAIN EIB 749 box, these new ETA 84XX tape encoders (Figure 1) promised to provide Keck Observatory true 4 mas (milliarcseconds) resolution in azimuth and 1 mas resolution in elevation. This was a big improvement to Keck Observatory’s position sensing when compared to the old rotary incremental encoders.
“In the end, these new HEIDENHAIN tape encoders performed brilliantly,” explained Krasuski. “Their installation required significant changes to our mechanical infrastructure in order to house them, so new designs were developed over time to get it just right.
“These totally new mechanical designs, plus the fact that it is very difficult to work at high altitude on the summit, made this an especially challenging project. It’s a two-hour ascent from our base to the summit, including spending time at an acclimatization zone each time someone is required to work there. And things operate differently up there since the atmospheric pressure is about 60 percent of sea level, making cooling systems tougher overall. There are many things to consider.”
Many telescopes utilize optical encoder tapes by placing the azimuth encoder tape on a dedicated stationary ring that is at or near the diameter of the azimuth bearing. Instead of doing this, the TCSU design team decided to pursue a shorter encoder tape on a dedicated ring located near the center of the telescope axis (Figure 2).
This ring has an O.D. of 1.15 meters and could accommodate an off-the-shelf full-circle HEIDENHAIN ERA 8400C encoder tape of 3.6 meters in length. This was a viable solution on K1 and K2 because of the existing accessory mirror mounting structure below the moving telescope tube. A connection to the telescope could be made here that would rotate an encoder in azimuth.